NAMEglCopyPixels - copy pixels in the frame buffer
CSPECIFICATION
void glCopyPixels( GLint x,
GLint y,
GLsizei width,
GLsizei height,
GLenum type )
PARAMETERSx, y Specify the window coordinates of the lower left corner
of the rectangular region of pixels to be copied.
width, height
Specify the dimensions of the rectangular region of
pixels to be copied. Both must be nonnegative.
type Specifies whether color values, depth values, or
stencil values are to be copied. Symbolic constants
GL_COLOR, GL_DEPTH, and GL_STENCIL are accepted.
DESCRIPTIONglCopyPixels copies a screen-aligned rectangle of pixels
from the specified frame buffer location to a region
relative to the current raster position. Its operation is
well defined only if the entire pixel source region is
within the exposed portion of the window. Results of copies
from outside the window, or from regions of the window that
are not exposed, are hardware dependent and undefined.
x and y specify the window coordinates of the lower left
corner of the rectangular region to be copied. width and
height specify the dimensions of the rectangular region to
be copied. Both width and height must not be negative.
Several parameters control the processing of the pixel data
while it is being copied. These parameters are set with
three commands: glPixelTransfer, glPixelMap, and
glPixelZoom. This reference page describes the effects on
glCopyPixels of most, but not all, of the parameters
specified by these three commands.
glCopyPixels copies values from each pixel with the lower
left-hand corner at (x + i, y + j) for 0<i<width and
0<j<height. This pixel is said to be the ith pixel in the
jth row. Pixels are copied in row order from the lowest to
the highest row, left to right in each row.
type specifies whether color, depth, or stencil data is to
be copied. The details of the transfer for each data type
are as follows:
GL_COLOR Indices or RGBA colors are read from the
buffer currently specified as the read source
buffer (see glReadBuffer). If the GL is in
color index mode, each index that is read
from this buffer is converted to a fixed-
point format with an unspecified number of
bits to the right of the binary point. Each
index is then shifted left by GL_INDEX_SHIFT
bits, and added to GL_INDEX_OFFSET. If
GL_INDEX_SHIFT is negative, the shift is to
the right. In either case, zero bits fill
otherwise unspecified bit locations in the
result. If GL_MAP_COLOR is true, the index
is replaced with the value that it references
in lookup table GL_PIXEL_MAP_I_TO_I. Whether
the lookup replacement of the index is done
or not, the integer part of the index is then
ANDed with 2b-1, where b is the number of
bits in a color index buffer.
If the GL is in RGBA mode, the red, green,
blue, and alpha components of each pixel that
is read are converted to an internal
floating-point format with unspecified
precision. The conversion maps the largest
representable component value to 1.0, and
component value 0 to 0.0. The resulting
floating-point color values are then
multiplied by GL_c_SCALE and added to
GL_c_BIAS, where c is RED, GREEN, BLUE, and
ALPHA for the respective color components.
The results are clamped to the range [0,1].
If GL_MAP_COLOR is true, each color component
is scaled by the size of lookup table
GL_PIXEL_MAP_c_TO_c, then replaced by the
value that it references in that table. c is
R, G, B, or A.
The GL then converts the resulting indices or
RGBA colors to fragments by attaching the
current raster position z coordinate and
texture coordinates to each pixel, then
assigning window coordinates (x +i,y +j),
where (x ,y ) is the current raster position,
and the pixel was the ith pixel in the jth
row. These pixel fragments are then treated
just like the fragments generated by
rasterizing points, lines, or polygons.
Texture mapping, fog, and all the fragment
operations are applied before the fragments
are written to the frame buffer.
GL_DEPTH Depth values are read from the depth buffer
and converted directly to an internal
floating-point format with unspecified
precision. The resulting floating-point
depth value is then multiplied by
GL_DEPTH_SCALE and added to GL_DEPTH_BIAS.
The result is clamped to the range [0,1].
The GL then converts the resulting depth
components to fragments by attaching the
current raster position color or color index
and texture coordinates to each pixel, then
assigning window coordinates (x +i,y +j),
where (x ,y ) is the current raster position,
and the pixel was the ith pixel in the jth
row. These pixel fragments are then treated
just like the fragments generated by
rasterizing points, lines, or polygons.
Texture mapping, fog, and all the fragment
operations are applied before the fragments
are written to the frame buffer.
GL_STENCIL Stencil indices are read from the stencil
buffer and converted to an internal fixed-
point format with an unspecified number of
bits to the right of the binary point. Each
fixed-point index is then shifted left by
GL_INDEX_SHIFT bits, and added to
GL_INDEX_OFFSET. If GL_INDEX_SHIFT is
negative, the shift is to the right. In
either case, zero bits fill otherwise
unspecified bit locations in the result. If
GL_MAP_STENCIL is true, the index is replaced
with the value that it references in lookup
table GL_PIXEL_MAP_S_TO_S. Whether the
lookup replacement of the index is done or
not, the integer part of the index is then
ANDed with 2b-1, where b is the number of
bits in the stencil buffer. The resulting
stencil indices are then written to the
stencil buffer such that the index read from
the ith location of the jth row is written to
location (x +i,y +j), where (x ,y ) is the
current raster position. Onlyrthe pixel
ownership test, the scissor test, and the
stencil writemask affect these write
operations.
The rasterization described thus far assumes pixel zoom
factors of 1.0. If
glPixelZoom is used to change the x and y pixel zoom
factors, pixels are converted to fragments as follows. If
(x , y ) is the current raster position, and a given pixel
isrin the ith location in the jth row of the source pixel
rectangle, then fragments are generated for pixels whose
centers are in the rectangle with corners at
(x +zoom i, y +zoom j)
r x r y
and
(x +zoom (i+1), y +zoom (j+1))
r x r y
where zoom is the value of GL_ZOOM_X and zoom is the value
of GL_ZOOM_Y. y
EXAMPLES
To copy the color pixel in the lower left corner of the
window to the current raster position, use glCopyPixels(0,
0, 1, 1, GL_COLOR);
NOTES
Modes specified by glPixelStore have no effect on the
operation of glCopyPixels.
ERRORSGL_INVALID_ENUM is generated if type is not an accepted
value.
GL_INVALID_VALUE is generated if either width or height is
negative.
GL_INVALID_OPERATION is generated if type is GL_DEPTH and
there is no depth buffer.
GL_INVALID_OPERATION is generated if type is GL_STENCIL and
there is no stencil buffer.
GL_INVALID_OPERATION is generated if glCopyPixels is
executed between the execution of glBegin and the
corresponding execution of glEnd.
ASSOCIATEDGETSglGet with argument GL_CURRENT_RASTER_POSITIONglGet with argument GL_CURRENT_RASTER_POSITION_VALIDSEEALSOglDepthFunc, glDrawBuffer, glDrawPixels, glPixelMap,
glPixelTransfer, glPixelZoom, glRasterPos, glReadBuffer,
glReadPixels, glStencilFunc